Question

When the time is negative? and why?
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Answer 1

Answer:

It sounds like you're using the equation

Δt=Δvaor equivalentlytf−ti=vf−via

Remember, the change denoted by Δ is always final (f) minus initial (i). Be careful about which is which.

If you are careful not to mix up the final and initial quantities, your answer will make sense.

By the way, scalars can be negative. The mere fact that Δt is a scalar doesn't mean it has to be positive. However, usually we say that final is later than initial, tf>ti, and that's why Δt is usually positive. It's good that you noticed that a negative time difference wouldn't make sense here.